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Workplace Health and Safety Queensland has issued a safety alert to help prevent the failure of zip lines, aiming to inform zip line owners, operators, engineers and riggers of the risk of failure of zip line terminations and to provide guidance to help prevent the failure of zip lines.

Defining a zip line as consisting "of a steel wire rope that is suspended between two points, along which one or more people travel while suspended from trolleys", the advisory note adds that "zip lines are also known as flying foxes".

The safety alert follow an incident in the Daintree Rainforest north of Cairns in October when an 86-metre zip line termination failed and two patrons dropped to the ground, causing fatal injuries to one and serious injuries to the other. The two patrons were travelling along the zip line, one behind the other, with a rope link between them.

It explains that rope terminations on zip lines generally use either of two basic types of rope termination:

• Pressed ferrule or swage type rope terminations
• Wire rope grips (e.g. bulldog grips)
• Both pressed ferrule terminations and wire rope grips rely on the application of pressure between two parallel sections of wire rope, the rope tail and the loaded section. The applied pressure creates frictional resistance to avoid failure of the termination by preventing relative movement.

It adds “over time the wire rope characteristics may change (e.g. the rope diameter may reduce) so that a termination which previously withstood applied loads may suddenly slip and cause failure of the zip line.

“Visual inspection of rope terminations may not be adequate on its own as a means for verifying the ongoing adequacy of terminations.”

The rope tension in zip lines can be extremely high and includes:

• Pre-tension applied to the rope during set up (to reduce sag, i.e. ‘catenary’)
• Tree movement
• The additional loads applied by patrons using the zip line.

The safety alert notes “it is important to note that loads applied by patrons to the zip line and its end anchorages will be many times their self-weight due to the triangulation effect of the load. For example, if a 100 kg patron uses a zip line, the additional tension applied to the zip line and its anchorages could be more than five times greater (i.e. potentially in excess of 500 kg force).”

Contributing factors
The incident is being investigated. It appears that the rope grip type termination at the top end of the zip line failed, allowing the zip line to disengage suddenly and causing both patrons to hit the ground.

Action required
Australian Standard AS 2316.2.1:20161 refers to the design of zip lines. There are several other Australian Standards relating to lifting and rigging gear that provide information on design and testing. The principles in these standards can also be applied to zip lines.

This safety alert provides information on zip line terminations and does not apply to other suspended rope applications.

Click here to read the full safety alert.

Image shows an example of how to calculate the proof test load for a zip line. Courtesy of Workplace Health and Safety Queensland.

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